The problem with flux is that it is corrosive. If you use just the flux in the solder it shouldn't be much of a deal, but if you use more flux I recommend washing it off.
I use 99.9% Isopropanol or an ultrasonic cleaner with water for this. If you use water do not use soap or anything else just water. Also make sure it is completely dry before connecting to power.
The encryption on Android devices is pretty strong, as long as you use a good screen lock you should be fine. Yes they can reset you phone, but accessing your data is a whole other level.
If I had illegal shit on my phone, I wouldn't send it to apple servers by using an iPhone. They are the first who would comply with a surpena. I'd use GrapheneOS on a Pixel and use an obvious duress pin like 1234. If entered it wipes your encryption keys and avoids restoring your data.
And if it gets stolen, it is gone and I'd get a new one. This is the cost of having proper opsec.
Edit:
But I also think that freedom allows for more exploits.
This is a common misconception called security through obscurity
Yes, opensource doesn't magically fix all vulnerabilities. But it is for sure way better then closed source, where you don't have a way of auditing the code
But this is the 3rd post